I can remember even Anand blundering/forgetting his preparation...along with other top GMs. I think it proves that they are only human. Carlsen is winning some of his games at Tata Steel 2013 by pressing them over 50 moves, and seeing the stress get the better of them...even from equal positions. They very well know that when they face him, no early draws like Anand and Kramnik...but 50, 60, 70 move games to test their end game skills.